1. Field of the Invention
The present invention relates to a styrene polymer composition comprising a styrene/n-butyl acrylate copolymer (I) and a branched block copolymer (II) comprising vinyl aromatic hydrocarbon monomer units and conjugated diene monomer units. More particularly, the present invention is concerned with a styrene polymer composition comprising a styrene/n-butyl acrylate copolymer (I) comprising styrene monomer units and n-butyl acrylate monomer units in a specific ratio, and a branched block copolymer (II) comprising vinyl aromatic hydrocarbon monomer units and conjugated diene monomer units in a specific ratio, wherein the branched block copolymer (II) has a block configuration comprising at least one vinyl aromatic hydrocarbon polymer block and at least one polymer block selected from the group consisting of at least one conjugated diene polymer block and at least one vinyl aromatic hydrocarbon/conjugated diene copolymer block, wherein the vinyl aromatic hydrocarbon polymer block of the branched block copolymer (II) exhibits, in a gel permeation chromatogram thereof, at least one peak molecular weight in each of two different specific ranges, wherein the branched block copolymer (II) has a specific vinyl aromatic hydrocarbon block ratio, and wherein the weight ratio of the styrene/n-butyl acrylate copolymer (I) to the branched block copolymer (II) is in a specific range.
A shaped article (such as a sheet or a film) obtained from the styrene polymer composition of the present invention not only exhibits high modulus in tension, high elongation at break, high surface impact strength, excellent transparency and excellent heat resistance, but also is free from marked occurrence of fish eyes (hereinafter, a xe2x80x9cfish eyexe2x80x9d is frequently referred to simply as an xe2x80x9cFExe2x80x9d). Further, by incorporating a specific additive into the styrene polymer composition of the present invention, the occurrence of FE""s in a shaped article (such as a sheet or a film) obtained therefrom can be more effectively suppressed. In addition, a heat shrinkable sheet obtained from the above-mentioned sheet not only is free from marked occurrence of FE""s but also exhibits excellent heat shrinkability at relatively low heating temperatures.
2. Prior Art
A block copolymer comprising vinyl aromatic hydrocarbon monomer units and conjugated diene monomer units wherein the vinyl aromatic hydrocarbon monomer unit content is relatively high, has various excellent characteristics, such as transparency and impact resistance, so that the block copolymer has been used for producing injection-molded products, extrusion-molded products (such as a sheet and a film) and the like. Further, a polymer composition comprising the above-mentioned block copolymer and a vinyl aromatic hydrocarbon/aliphatic unsaturated carboxylic acid derivative copolymer has excellent transparency, mechanical properties and shrinkability, so that the composition has been used for producing a sheet, a film and the like.
Conventionally, with respect to the above-mentioned polymer composition, and a sheet or film obtained therefrom, a number of proposals have been made in patent documents. For example, as a composition having not only high tensile strength, modulus and impact strength, but also excellent optical characteristics, elongation property and cracking resistance and the like, Unexamined Japanese Patent Application Laid-Open Specification No. 59-221348 discloses a polymer composition comprising a vinyl aromatic hydrocarbon/aliphatic unsaturated carboxylic acid derivative copolymer and a copolymer comprising at least one vinyl aromatic hydrocarbon polymer block and at least one polymer block composed mainly of a conjugated diene, wherein the vinyl aromatic hydrocarbon/aliphatic unsaturated carboxylic acid derivative copolymer has an aliphatic unsaturated carboxylic acid derivative content of from 5 to 80% by weight and has a Vicat softening point of 90xc2x0 C. or less. As a heat shrinkable film having excellent heat shrinkability at relatively low heating temperatures, optical characteristics, cracking resistance, dimensional stability and the like, Unexamined Japanese Patent Application Laid-Open Specification No. 61-25819 discloses a film obtained by stretching a film of a polymer composition comprising a vinyl aromatic hydrocarbon/aliphatic unsaturated carboxylic acid derivative copolymer and a copolymer comprising at least one vinyl aromatic hydrocarbon polymer block and a polymer block composed mainly of a conjugated diene derivative, wherein the vinyl aromatic hydrocarbon/aliphatic unsaturated carboxylic acid derivative copolymer has a vinyl aromatic hydrocarbon content of from 20 to 95% by weight and has a Vicat softening point of 90xc2x0 C. or less. Further, as a transparent, heat shrinkable film having excellent resistance to deterioration with time and impact resistance, Unexamined Japanese Patent Application Laid-Open Specification No. 5-104630 discloses a rigid, heat shrinkable film of a polymer composition comprising a vinyl aromatic hydrocarbon/aliphatic unsaturated carboxylic acid derivative copolymer and a copolymer comprising at least one vinyl aromatic hydrocarbon polymer block and at least one polymer block composed mainly of a conjugated diene derivative, wherein the vinyl aromatic hydrocarbon/aliphatic unsaturated carboxylic acid derivative copolymer has a Vicat softening point of 105xc2x0 C. or less. This heat shrinkable film exhibits a heat shrinkability in a specific range, as measured-in a specific direction of the film. Further, as a polymer composition having excellent transparency, stiffness, and surface impact resistance at low temperatures, Unexamined Japanese Patent Application Laid-Open Specification No. 6-220278 discloses a polymer composition comprising a block copolymer comprising a vinyl aromatic hydrocarbon and a conjugated diene and a vinyl aromatic hydrocarbon/(meth)acrylic ester copolymer resin, wherein the block copolymer has a specific block configuration and a specific molecular weight distribution. Moreover, as a resin composition having excellent transparency and impact resistance, Unexamined Japanese Patent Application Laid-Open Specification No. 7-216187 discloses a transparent resin composition having high strength, which comprises a block copolymer and a vinyl aromatic hydrocarbon/(meth)acrylic ester copolymer, wherein the block copolymer comprises two vinyl aromatic hydrocarbon polymer blocks and two vinyl aromatic hydrocarbon/conjugated diene copolymer blocks.
However, each of the conventional polymer compositions described in the above-mentioned patent documents, each of which comprises a block copolymer comprising vinyl aromatic hydrocarbon monomer units and conjugated diene monomer units and a vinyl aromatic hydrocarbon/aliphatic unsaturated carboxylic acid derivative copolymer, has a problem in that, when the conventional composition is used for producing a relatively thin sheet or film, it is impossible to obtain a sheet or film which not only has high modulus-in-tension, high elongation at break, high surface impact strength, excellent transparency and excellent heat resistance, but also is free from marked occurrence of FE""s. In addition, the above-mentioned patent documents do not have any description about how to solve the problem.
In this situation, the present inventors have made extensive and intensive studies with a view toward solving the above-mentioned problem accompanying the prior art, i.e., with a view toward developing a block copolymer composition which can be advantageously used for producing a shaped article (such as a sheet or a film) which not only has high modulus-in-tension, high elongation at break, high surface impact strength, excellent transparency and excellent heat resistance, but also is free from marked occurrence of FE""s. As a result, it has unexpectedly been found that a desired composition having the above-mentioned excellent properties can be realized by a styrene polymer composition comprising a styrene/n-butyl acrylate copolymer (I) comprising styrene monomer units and n-butyl acrylate monomer units in a specific ratio, and a branched block copolymer (II) comprising vinyl aromatic hydrocarbon monomer units and conjugated diene monomer units in a specific ratio, wherein branched block copolymer (II) has a block configuration comprising at least one vinyl aromatic hydrocarbon polymer block and at least one polymer block selected from the group consisting of at least one conjugated diene polymer block and at least one vinyl aromatic hydrocarbon/conjugated diene copolymer block, wherein the vinyl aromatic hydrocarbon polymer block of the branched block copolymer (II) exhibits, in a gel permeation chromatogram thereof, at least one peak molecular weight in each of two different specific ranges, wherein the branched block copolymer (II) has a specific vinyl aromatic hydrocarbon block ratio, and wherein the weight ratio of the styrene/n-butyl acrylate copolymer (I) to the branched block copolymer (II) is in a specific range. Based on this finding, the present invention has been completed.
Accordingly, it is a primary object of the present invention to provide a styrene polymer composition which can be advantageously used for producing a shaped article (such as a sheet or a film) which not only has high modulus-in-tension, high elongation at break, high surface impact strength, excellent transparency and excellent heat resistance, but also is free from marked occurrence of FE""s.